1. Field of the Invention
This invention relates to the use of cameras for monitoring a remotely located welding operation, particularly the automatic adjustment of light incident upon the camera to reduce glare from the welding arc for better contrast resolution and clearer imaging.
2. Description of the Related Art including information disclosed under 37 C.F.R. .sctn..sctn. 1.97-1.99
The arc welding machines used for welding pipes of large dimensions are the orbital type and incorporate a motorized support member mounted for movement on a circular rail fixed on one of the pipes being joined. On this support member is mounted a welding head incorporating an electrode permitting the refusion of filler wire continuously fed to the zone of the arc struck between the electrode and the junction zone of the parts. The welding head is mounted for oscillation in the transverse direction in relation to the orbital movement of the welding machine. A sweep is thus made over the width of the weld bead.
Systems for video monitoring an arc welding operation are known in the prior art. Such systems are often used in situations where it is impossible or impractical for a human operator to have direct, hands-on access to the workpiece such as when a weld must be produced in the inner diameter of a pipe, or within the radioactive environment of the steam pipes of a nuclear power generator. These video monitoring systems allow the welder to observe the position of the electrode and the filler wire relative to the workpiece, as well as the characteristics of the weld puddle created by the electric arc. This information helps the operator to remotely control the dynamics of the weld puddle so that the weld is produced uniformly and without flaws.
To lengthen the life of a nuclear power station, it has been proposed to use new piping systems to replace older piping systems and other components which, due to many factors, are deemed to have reached the end of their useful life.
To replace the whole of a steam generator, it is in particular necessary to join, by welding, the primary water inlet and outlet connections of the steam generator to the corresponding primary circuit pipes held ready in the reactor building. This operation can be carried out by a semi-automatic welding operation controlled from a control station situated a certain safe distance from the welding zone properly shielded against nuclear radiation. In order to ensure proper welding throughout the operation, an operator must permanently monitor the welding operation with the aid of a visual display, on a television screen, of the zone in which the weld bead is formed.
It is necessary to provide a very clear image of the welding zone, because only with the aid of the image on the screen can the operator evaluate any action that must be taken to achieve the deposition of a perfect weld bead. It is in fact necessary to obtain a deposit of weld metal which is free from defects as defective welds can lead to expensive re-work or failure. The high temperatures that are used to weld two pieces of material together, typically two metal pieces, are accompanied by high light intensities creating dramatic contrasts between lit and dark areas. This coupling of high temperature with bright light creates problems when the welding area requires close inspection during the welding process.
Originally, welders used masks with plates of heavily darkened glass in order to observe the welding area. However, remote controlled welding in hazardous environments, such as nuclear facilities, have become more commonplace. For these hazardous environments, remote controlled welding machines have been developed that allow a welder to control the welding operation from a safe environment.
In order to observe the welding procedure from a safe environment, video cameras are used to view the welding operation in order to monitor the progress and quality of the weld. Such cameras may be n"charge coupled devices" or CCD's and allow the welder to observe the welding environment under normal conditions. However, when the welding arc is lit, it emits a relatively broad spectrum of light, including light within the infrared, visible, and ultraviolet spectrums.
The images produced by such video monitoring systems are generally not as clear and informative to a welder as the image he might see through a conventional welding hood. This difference in quality stems largely from the fact that the sensing circuit of a conventional television camera (which is a pixel array of a light-sensitive charge coupled device) does not have the interpretative abilities that the human eye has when used in combination with the brain. While the filter glass used in a conventional welding hood passes an image (where the arc is essentially white and the background of the workpiece is very nearly black) the eye and brain are able to interpret this image so that the welder is able to see fairly clearly the position of the electrode of the torch relative to the workpiece, the resulting weld puddle, the contour of the weld, and the wettability of the leading edge of the puddle.
In the case of a television camera, the bright image of the arc saturates a portion of the charge coupled device (CCD) of the light sensing circuit, yielding a television image that includes severe contrasts wherein the back-ground surrounding the electric arc is darkened into obscurity. The emission of visible light from the welding torch is so intense that the image transmitted by the CCD is not clear. The arc and its proximate plasma overloads the video camera, obscuring the weld puddle melted by the welding torch. Additionally, the light filtration components in these systems may create "ghost" images and stray refractions which further blur the resulting image.
The problem presented when monitoring remotely controlled welding operations is how to observe the welding operation in the glare of the welding torch that obscures a view of the weld puddle formed between the surfaces being welded together.
In an attempt to balance these severe contrasts between the welding area with the arc torch and the surrounding area, some prior art television systems have employed pulsed light sources such as xenon flash units or lasers synchronized with the actuation of the television camera, in combination with narrow bandwidth filters which conduct only a very narrow range of the light frequencies present in the spectrum of the xenon or laser light. As only the pulsed light is allowed through the filter, the light from the arc is eliminated and the welding area is viewed by illumination by the pulsed light. Unfortunately, only a monochrome image is obtained as only the filtered light is used to form an image. Also, the use of such background lighting requires the positioning of still another component in the often limited access area around the weld, and increases the cost of the operation of the monitoring system.
Televisual observation of the welding zone is therefore complicated by the existence of a very bright spot or zone, corresponding to the arc moving over the width of the weld bead, at the center of the welding zone. The weld bead on which the operator must concentrate his attention may be poorly visible because of the very high luminous intensity of the arc.
In the prior art, a semi-opaque filter in the form of a small dot placed strategically between the CCD camera and the arc of the welding torch has been used. The small dot performs much like the hand of a person placed to block the sun when looking at a plane in the sky on a sunny day. The small dot casts a shadow upon the CCD effectively obstructing the glare emitted directly from the welding torch. While this technique is partially effective, drawbacks remain.
Foremost of these drawbacks is the obstruction of the scene that is of most interest in the welding operation, the activity at the tip of the welding torch. Dirt or other foreign matter cannot be seen in the weld when obstructed by the dot. Neither can the weld puddle, filler wire entry or other processes at the arc tip be seen. Other details in the critical welding area are likewise obscured. The static nature of the dot's position requires the camera to display the same scene relative to the welding arc so that the dot may continually block the intense light emitted from the welding arc. Other views are not permitted. Also, the parts of the welding zone which are not directly subjected to the light of the ar will then be poorly visible, even if floodlights are used to illuminate the welding zone.
To achieve effective monitoring of the welding in progress, the operator must be provided with an image of the entire welding zone which is sufficiently clear, in particular, for verification of the state of the bead just deposited and its position in relation to the other beads deposited previously, and also for monitoring the progress of the welding.
In addition to the problem of obscuring contrasts, a second major problem associated with many prior art systems is the mechanical interference that the components of the video monitoring system impose upon the movement of the weld head. In some systems, the movement of the weld head is limited by a relatively large and bulky camera assembly which is connected directly on the weld head. The relatively large and bulky size of such camera assembly is due in part to the fact that such assemblies must contain not only the television camera, but a separate array of lenses, electronic light valves, iris diaphragms, neutral density filters or cross-polarizing filters all positioned in tandem which are used to dim the image of the arc before it strikes the light-sensitive array of charge coupled devices within the camera. In an attempt to solve the mechanical interference problems caused by the mounting of the camera assembly on the weld head, some other prior art systems use fiber optical cables to remotely transmit an optical image to a television camera. However, these fiber optical cables lose a great deal of the transmitted light which reduces the resulting resolution of the television image, and are easily browned and burned out by nuclear radiation which in turn requires costly frequent replacement.
The bulky nature of previous systems was only compounded by the addition of extra equipment allowing the welding arc to be blocked in a movable or dynamic manner or, to move the blocking spot that obscured the welding arc. Previous systems designed to block the overwhelming glare of the welding arc were expensive, limiting their availability in a variety of
Clearly, there is a need for a television monitoring system for observing a welding operation which produces a clear, sharp and contrast-balanced image where the portion of the workpiece surrounding the welding arc is easily visible without the need for auxiliary background lighting. Ideally, such a system should be compact enough in design so as to offer little or no mechanical interference to the weld head as it is manipulated during a welding operation. Moreover, the system should be comprised of a small number of simple and inexpensive parts that are capable of producing a clear and well lighted welding image under a variety of different conditions and for a variety of different welding systems. There is a need for dynamic optical filtration of the welding arc glare so that the camera monitoring the welding process could be moved and positioned for the best view without limitation due to the position of the welding arc relative to the camera.